An image sensor captures an image by using a semiconductor device's feature of reacting to external energy, for example, photons. Light generated from an object existing in nature has an intrinsic energy value in a wavelength thereof. A pixel of the image sensor detects the light generated from the object and converts the detected light to an electric value.
A photodiode included in the pixel of the image sensor typically has a P-I-N (P-type semiconductor-intrinsic semiconductor-N-type semiconductor) structure to maximize a quantum efficiency. The advantage of the P-I-N structure lies in that a wide depletion region can be formed by injecting impurity having a concentration lower than other N-type region. A strong electric field can be generated in the depletion region. Thus, the pairs of electrons and holes generated by photons incident on the depletion region are not recombined but separated from one another. That is, since the electrons enter in the photodiode while the holes are exhausted out of the photodiode, the electrons are completely captured. However, a weak electric field is generated in a balance region, other than the depletion region. Thus, since the electron and hole pairs generated by the photons incident on the balance region are highly likely to recombine, a possibility of capturing the electrons, that is, the quantum efficiency, is remarkably lowered. As a result, to improve the quantum efficiency, it is advantageous to increase the size of the depletion region as well as the size of the photodiode.
FIG. 1 shows a conventional 3-transistor CMOS active pixel, in which the section of a photodiode is shown with electronic signs of other elements. In the conventional 3-transistor CMOS active pixel, an N− type impurity layer 11 constituting one side conjunction of the photodiode contacts an N type floating diffusive layer 13. That is, since the floating diffusive layer 13 functions as a capacitor of the photodiode, an actual capacitance component of the photodiode is a sun of capacitor components generated by the N? type impurity layer 11 and the N type floating diffusive layer 13. Thus, the sensitivity of the image sensor employing the conventional 3-transistor CMOS active pixel is deteriorated.
Also, as the N type impurity of the floating diffusive layer 13 enters the N? type impurity layer 11, the size of the depletion region decreases. Thus, even if an externally input voltage is increased, a critical voltage, that is, a pining voltage VPIN, at which the depletion region is not formed any more, can be lower than an external power voltage VDD. In this case, however, the operating range of the CMOS active pixel decreases.